Variable Order Fractional Controllers

This paper addresses variable order fractional controllers. Four situations where variable order fractional controllers may be used to cope with time-varying plants are used as examples. In all cases a constant phase margin is sought, thus resulting in a constant overshoot in step responses, which is otherwise unattainable. The first two examples concern first generation Crone controllers for plants with time-varying gains. The other two examples concern a logarithmic phase Crone controller for a plant with time-varying poles and gain. Numerical simulation results are given for all examples.